The present invention relates to a magnetic developer comprising magnetic toner particles and magnetic carrier particles in mixture, which is used in the development of electrostatic images formed on the surface of an image carrier by magnetic brushing, and more particularly to a magnetic developer having better triboelectric characteristics and a highly stabilized image density in the continuous development.
In the development of electrostatic images formed on the surface of an image carrier, magnetic brushing is generally used. In most cases, a two-components-based developer comprising magnetic carrier particles and non-magnetic toner particles in mixture is used as a developer for the magnetic brushing development. On the other hand, a single component-based developer comprising a resin and magnetic powder is also used as the developer. The development with the former two-components-based developer can produce images with a good image concentration and a good resolution, but has a poor half-tone reproducibility.
The development with the latter single component-based developer has such problems as occurrence of aggregation of toner particles by electrocharging due to an increase in the charged quantity of toner particles and poor development due to shortage of toner particles on the sleeve. To solve these problems, for example, U.S. Pat. No. 4,640,880 and U.S. Pat. No. 5,053,305 propose magnetic developers each based on a mixture of magnetic carrier particles and magnetic toner particles, stating that the proposed magnetic developers have advantages of both of the conventional two-components-based developer and single component-based developer.
Development of electrostatic images with the above-mentioned developer based on a mixture of magnetic carrier particles and magnetic toner particles has advantages of the developments with the two-component-based developer and also with the single component developer, but when the magnetic toner particles have a lower coercive force or when the magnetic toner particles contain less magnetic powder, it has a problem of no sharper developed images. That is, due to poor transportability of the developer by a non-magnetic sleeve positioned against the surface of an image carrier and provided with a magnetic field-generating member such as a permanent magnet, etc. inside, makeup for the magnetic toner particles consumed by sliding friction on the surface of the image carrier by a magnetic brush will be short, resulting in a higher concentration of magnetic carrier particles.
In solving the problem, it is not desirable to simply increase the number of revolutions per minute of the sleeve or the magnetic field-generating member as a means of supplying the magnetic developer, because this considerably deteriorates office atmospheres due to generation of noises, etc., and it is not acceptable to increase the outer diameter of the sleeve or the magnetic field-generating member, because this is against the recent requirements for smaller size, thinner type and smaller weight of the developing apparatuses.
Furthermore, the magnetic toner particles that constitute a magnetic developer contain a charge-controlling agent such as a nigrosine dye or a metal-containing azo dye to give a sufficient changeability, that is, to control the charging. By addition of the charge-controlling agent, the changeability of the magnetic developer can be increased and the image density can be improved. That is, the magnetic toner particles are held on the surfaces of magnetic carrier particles by magnetic attractive forces on the way to the development zone by the magnetic transportation means, and thus the deposition force of magnetic toner particles onto the photosensitive member by an electrostatic force in the development zone can be increased by improving the chargeability of magnetic toner particles, thereby improving the image density.
In the above-mentioned, conventional two-components-based developer, holding of the magnetic carrier particles and the non-magnetic toner particles is attained only by the electrostatic force, and thus the improvement of the chargeability of non-magnetic toner particles causes to increase the deposition force of the magnetic carrier particles and the non-magnetic toner particles in the development zone, resulting in a decrease in the image density.
To improve the chargeability of magnetic toner particles, it is effective to contain the charge-controlling agent, as mentioned above, but the charge-controlling agent has a low compatibility with a binder resin that constitutes the magnetic toner particles, and a poor dispersibility. That is, there is a limit to the content of the charge-controlling agent, and it is difficult to obtain the necessary chargeability. Thus, the chargeability is low just after the start of the developing apparatus, and it takes a long time until the desired chargeability is obtained, that is, the rise time is prolonged and there is also such a problem as a lowered image density in the continuous development.